This project is directed toward correlation of the ultrastructure and the physiology of the lateral compound eye of Limulus, its lateral plexus and neuropile. Serial representation of the lateral compound eye from both adult and immature animals are being obtained. From these serial reconstructions we are attempting to determine the interrelationships between components of the eye and to correlate these data with data obtained by neurophysiological methods. We are also comparing the ultrastructure of the immature animals with that of adults. Similar ultrastructural studies are planned for developmental stages of Limulus eye. Special attention is being paid to our effort to develop an intracellular electron stain which affords fine resolution of the collateral axon branches that comprise the complex neuropile.